1. Technical Field
The invention relates to optical devices and is especially applicable to waveguide structures and integrated optics.
2. Background Art
This specification refers to several published articles. For convenience, the articles are cited in full in a numbered list at the end of the description and cited by that number in the specification itself. The contents of these articles are incorporated herein by reference and the reader is directed to them for reference.
At optical wavelengths, the electromagnetic properties of some metals closely resemble those of an electron gas, or equivalently of a cold plasma. Metals that resemble an almost ideal plasma are commonly termed xe2x80x9cnoble metalsxe2x80x9d and include, among others, gold, silver and copper. Numerous experiments as well as classical electron theory both yield an equivalent negative dielectric constant for many metals when excited by an electromagnetic wave at or near optical wavelengths [1,2]. In a recent experimental study, the dielectric function of silver has been accurately measured over the visible optical spectrum and a very close correlation between the measured dielectric function and that obtained via the electron gas model has been demonstrated [3].
It is a well-known fact that the interface between semi-infinite materials having positive and negative dielectric constants can guide TM (Transverse Magnetic) surface waves. In the case of a metal-dielectric interface at optical wavelengths, these waves are termed plasmon-polariton modes and propagate as electromagnetic fields coupled to surface plasmons (surface plasma oscillations) comprised of conduction electrons in the metal [4].
A metal film of a certain thickness bounded by dielectrics above and below is often used as an optical slab (planar, infinitely wide) waveguiding structure, with the core of the waveguide being the metal film. When the film is thin enough, the plasmon-polariton modes guided by the interfaces become coupled due to field tunnelling through the metal, thus creating supermodes that exhibit dispersion with metal thickness. The modes supported by infinitely wide symmetric and asymmetric metal film structures are well-known, as these structures have been studied by numerous researchers; some notable published works include references [4] to [10].
In general, only two purely bound TM modes, each having three field components, are guided by an infinitely wide metal film waveguide. In the plane perpendicular to the direction of wave propagation, the electric field of the modes is comprised of a single component, normal to the interfaces and having either a symmetric or asymmetric spatial distribution across the waveguide. Consequently, these modes are denoted sb and ab modes, respectively. The sb mode can have a small attenuation constant and is often termed a long-range surface plasmon-polariton. The fields related to the ab mode penetrate further into the metal than in the case of the sb mode and can be much lossier by comparison. Interest in the modes supported by thin metal films has recently intensified due to their useful application in optical communications devices and components. Metal films are commonly employed in optical polarizing devices [11] while long-range surface plasmon-polaritons can be used for signal transmission [7]. In addition to purely bound modes, leaky modes are also known to be supported by these structures.
Infinitely wide metal film structures however are of limited practical interest since they offer 1-D field confinement only, with confinement occurring along the vertical axis perpendicular to the direction of wave propagation implying that modes will spread out laterally as they propagate from a point source used as the excitation. Metal films of finite width have recently been proposed in connection with polarizing devices [12], but merely as a cladding.
The present invention seeks to eliminate, or at least mitigate, the disadvantages of the prior art.
According to the present invention there is provided a waveguide structure comprising a thin strip of a material having a relatively high free charge carrier density surrounded by a material having a relatively low free charge carrier density, the strip having finite width and thickness with dimensions such that optical radiation having a wavelength in a predetermined range couples to the strip and propagates along the length of the strip as a plasmon-polariton wave.
Such a strip of finite width offers 2-D confinement in the transverse plane, i.e. perpendicular to the direction of propagation, and, since suitable low-loss waveguides can be fabricated from such strip, it may be useful for signal transmission and routing or to construct components such as couplers, power splitters, modulators and other typical components of integrated optics.
For example, where the optical radiation has a free-space wavelength of 1550 nm, and the waveguide is made of a strip of a noble metal surrounded by a good dielectric, say glass, suitable dimensions for the strip are thickness less than about 0.1 microns, preferably 20 nm, and width of a few microns, preferably about 4 microns.
The strip could be straight, curved, bent, tapered, and so on.
The dielectric material may be inhomogeneous, for example a combination of slabs, strips, laminae, and so on. The conductive or semiconductive strip may be inhomogeneous, for example a gold layer sandwiched between thin layers of titanium.
The plasmon-polariton wave which propagates along the structure may be excited by an appropriate optical field incident at one of the ends of the waveguide, as in an end-fire configuration, and/or by a different radiation coupling means.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description, in conjunction with the accompanying drawings, of a preferred embodiment of the invention.